1. Field of the Invention
The invention is related to the field of temperature sensors, and more particularly, to temperature sensors including multiple elements.
2. Description of the Prior Art
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. When there is no flow through the flowmeter, all points along a flow tube oscillate with identical phase. As the material begins to flow, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver.
Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Frequency of the vibrational response may be measured by a single transducer, wherein the frequency is used to determine the density of material in the meter. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things. Vibratory flowmeters, including Coriolis mass flowmeters and densitometers, therefore employ one or more flow conduits that are vibrated in order to measure a fluid.
Fluid flow though a flow tube creates only a slight phase difference on the order of several degrees between the inlet and outlet ends of an oscillating flow tube. When expressed in terms of a time difference measurement, the phase difference induced by fluid flow is on the order of tens of microseconds down to nanoseconds. Typically, a commercial flow rate measurement should have an error of less than one-tenth of one percent. Therefore, a flow meter must be well designed in order to accurately measure these slight phase differences.
The vibrational characteristics of the vibrating structure change with changes in temperature. The vibrating flow tube(s) are typically formed from a metallic material having a Young's modulus that changes with temperature. In order to maintain high measurement accuracy, the temperature of the vibrating structure is typically measured and compensation is made for the change in Young's modulus with changes in temperature.
A Coriolis flowmeter system is comprised of two components; a flowmeter element and a transmitter. The flowmeter element is the actual sensor, containing vibrating tube(s), through which fluid flows while the transmitter is the signal processing device that receives and processes signals from the flowmeter element. Electrical connections between the flowmeter element and the transmitter are made over a multi-conductor cable. The shielded cable is comprised of a shielded conductor pair for providing a drive signal to the driver, second and third shielded conductor pairs for transmitting signals from the pick-off sensors and a shielded conductor triplet for transmitting a signal from a temperature sensor located on the vibrating flow tube. A three wire temperature sensor is typically used since this allows for a compensation of the resistance in the cable between the flowmeter element and the flowmeter transistor. This nine wire cable is not a standard cable in the process control industry. Therefore, each time a Coriolis flowmeter is installed using a transmitter mounted remotely from the flowmeter element, a special, non-standard cable must be run between the flowmeter element and the transmitter. This creates additional expense.
As flow meter technology develops, the performance demands (and changes to the geometry of the vibrating flow tubes) have brought about a need to make temperature measurements at multiple points on the flow meter element. A temperature measurement of the vibrating structure and a temperature measurement of the non-vibrating structure may be needed. Alternatively, a temperature measurement of a wetted portion of the vibrating structure and a temperature measurement of a non-wetted portion of the vibrating structure might be necessary. In any event, when more than one temperature sensor is used in existing Coriolis flow meter designs, conductors in addition to those available in the typical nine wire cable used with Coriolis flow meters are required. A cable having more than the traditional nine conductors is a problem for several reasons. One reason is that even the existing nine wire cable is expensive. Using a cable with even more conductors adds additional expense. Therefore, regardless of the number of temperature sensors, it is advantageous to minimize the number of conductors. Additional conductors in the cable require additional connectors on both the flowmeter element and the transmitter. This adds additional cost and can pose problems if there is not enough physical space for the additional connectors. This is particularly true for intrinsically safe applications.
Another reason why adding additional conductors to the cable is a problem is one of compatibility. Manufacturers incur additional expense and complexity where different types of flowmeter models require different cables. Also, there exists a large installed base of Coriolis flowmeters using nine wire cables. New flow meter designs can replace old flowmeters if the same cable is used.
There exists a need for a temperature sensor system that provides for multiple temperature sensors while minimizing the number of conductors between the flow meter element and the transmitter. There exists a further need for a flow meter employing two temperature sensors that utilizes the existing nine wire cable typically used with Coriolis flow meters.
FIG. 1 shows a Coriolis mass flow meter 5 comprising a meter assembly 10 and a meter electronics 20 coupled to the meter assembly 10 via a multi-conductor cable 100. The meter electronics 20 may provide density, mass flow rate, volume flow rate and/or temperature data over the path 26. A Coriolis flow meter structure is described although it is apparent to those skilled in the art that the present invention could alternatively comprise a vibrating tube densimeter 5.
The meter assembly 10 includes a pair of flanges 101 and 101′ and corresponding manifolds 102 and 102′. Fluid enters the meter assembly 10 through one of the flanges 101 or 101′ and passes through flow tube 103, leaving the meter assembly 10 through the other flange 101′ or 101.
The flow tube 103 is encircled by a balance tube 104. The flow tube 103 is connected to the balance tube 104 and the balance tube 104 is connected to the case ends 105 and 105′. The case ends 105 and 105′ form the end of the case 106.
The figure illustrates a straight flow tube 103, but those skilled in the art will recognize that the present invention can be applied to a flow meter system having a flow tube of any geometry. Also, a flow element having multiple flow tubes through which fluid flows is clearly within the scope of the present invention.
A driver 107 is connected to the balance tube 104 at the mid-point of the balance tube 104. One or more pick-off sensors 108 and 108′ are connected to the balance tube 104 and the flow tube 103. In one embodiment of the present invention, each of the pick-off sensors 108 and 108′ comprises a coil attached to the balance tube 104 and a magnet attached to the flow tube 103 and formed to move within the magnetic field that is generated when a periodic signal is applied to the coil. Those skilled in the art recognize that pick-off sensors of any design could be used, e.g., accelerometers or potentiometers, and that the velocity sensors described are merely exemplary.
A counterbalance 115 may be connected to the balance tube 104 diametrically opposite of the driver 107. The mass of the counterbalance 115 is determined by the density of the expected process fluid to be measured by the flow meter system 5. A flow tube temperature sensor 109 is attached to the flow tube 103 and a balance tube temperature sensor 110 is attached to the balance tube 104.
Cable 100 is comprised of a conductor 111 which carries the drive signal from the meter electronics 20 to the driver 107, conductors 112-113 which carry the pick-off signals from the left and right pick-off sensors 108 and 108′ to the meter electronics 20, respectively, and a conductor 114 which carries temperature sensor information to the meter electronics 20. The conductors 111-113 may each comprise two conductors and the conductor 114 may comprise three separate conductors, such that cable 100 comprises nine component conductors.
The operation of the meter electronics 20 to produce mass flow rate, volume flow rate and density information is well known to those skilled in the art of flow measurement and does not form part of the present invention. The circuitry including the flow tube temperature sensor 109, balance tube temperature sensor 110, and the conductor 114 and the associated circuitry within the meter electronics 20 form the basis for the remaining description.
It is known to those skilled in the art that the Coriolis flow meter system 5 is quite similar in structure to a vibrating tube densitometer. Vibrating tube densitometers also utilize a vibrating tube through which fluid flows or, in the case of a sample-type densitometer, within which fluid is held. Vibrating tube densitometers also employ a drive system for exciting the flow tube to vibrate. Vibrating tube densitometers typically utilize only a single feedback signal, i.e., from a single pick-off, since a density measurement requires only the measurement of frequency and a phase measurement is not necessary. The descriptions of the present invention herein apply equally to vibrating tube densitometers.
FIG. 2 shows a prior art resistive temperature network. The temperature sensor portion comprises the resistors or resistive temperature devices (RTDs) 110 and 109. A direct current (DC) voltage is provided through the switch (F0) and creates electrical currents through the temperature resistors, among others. The resulting electrical currents create voltage drops across the temperature sensing resistors. The voltage drops will vary with the resistance of the temperature sensing resistors, which will in turn be dependent on temperature. The voltages across the resistors 110 and 109 are sensed and used to determine the corresponding temperatures at the resistors 110 and 109.
A disadvantage in the prior art is that multiple measurements are required in order to read the values of the resistive elements 110 and 109. The prior art circuit requires eight voltage measurements; four voltage measurements with the switch (F0) on, and four more voltage measurements with the switch (F0) off.
Another disadvantage in the prior art is that a multiplexer (MUX) is needed in order to switch between various DC voltages in the resistive network for the purpose of making the required voltage measurements. Consequently, each of the eight voltages must be sequentially connected to the voltage-to-frequency (V/F) converter and digitized/measured. Temperature is measured serially, with each voltage measurement occurring at a successive time period, wherein a temperature measurement requires eight such measurement periods. Therefore, where the temperature may be changing rapidly, there is a delay in measuring the voltages and the resulting temperature value may not be up to date.
What is needed, therefore, are improvements in measuring temperature using a temperature network.